Eighteen months before her death, the children’s writer Rosemary Sutcliff told The TES that if she only had a few hours to live, she would go on writing until the last minute, even though the book would never be finished. “It’s an affirmation of life, isn’t it?” When she died suddenly in July 1992 she was two-thirds of the way through the second draft of Sword Song, which will be published by The Bodley Head next month.

The manuscript was discovered among her papers by her godson and literary executor, Anthony Lawton. He explains that it has taken five years to publish because he felt that he first had to recover from her death. “She was more than just a godmother; I spent a lot of my childhood wheeling her about, sometimes accidentally tipping her into the flowerbeds. She also sponsored my education. ”

Also, because she always made three drafts of her books in a tiny “spidery longhand”, transcribing Sword Song took some time.

Jill Black, who had been Ms Sutcliff’s editor for 30 years, made the final decision about whether Sword Song should be published. She was anxious to make sure that the book was worthy of Rosemary Sutcliff and not an unfinished fragment that its author would have been embarrassed to see in print. “But when I read it I was convinced that it stands up well beside her earlier work, ” she said. Ms Black, who was also Graham Greene’s editor, said that Rosemary Sutcliff never needed much editing and would never discuss her books until she had completed her final version.

Sword Song tells the story of Bjarni Sigurdson, a 16-year-old Viking boy who seeks his fortune as a mercenary swordsman among the Viking settlers and native Britons and Celts of Western Scotland and Ireland. Like most of her books, it has a strongly masculine flavour, being full of action, fights and shipwrecks and a powerful, though never obtrusive, moral undertone - Bjarni’s adventure is also a journey to maturity.

All Rosemary Sutcliff’s books - more than 50 of them - were meticulously researched, says Jill Black. But most of her fans will attest that no amount of research could account for her uncanny ability to capture the essential flavour of the historical periods in which the stories are set. It is almost as if she had lived through them and was simply recounting events from her past. In an interview with The TES in November 1990, she confessed to a tentative belief in reincarnation.

Her ability to write convincingly about the exploits of very active young men in Roman Britain (in tales such as The Eagle of the Ninth) was particularly remarkable considering that she spent most of her life in a wheelchair, having suffered since childhood from a vicious form of arthritis.

Anne McNeill, Rosemary Sutcliff’s publisher, says that her books still sell steadily, although they are perceived as demanding by some teachers and librarians because of their unfamiliar settings and their length. “But this one, with its strong regional flavour, should grip those who are not usually attracted to historical novels.” She also suggested that it could make a good film, “a Viking Braveheart”.

Rosemary Sutcliff said herself that she wrote for “children between the ages of eight and 80”.

Sword Song will be reviewed in The TES on July 4.